1. Technical Field
This invention relates generally to one-way clutches, and more particularly to the manufacture of the races for such clutches.
2. Related Art
The fundamental operating principle of one-way clutches is well known to those of ordinary skill in the art. Relative rotation of inner and outer elements is prevented in one direction, enabling the transmission of torque across the clutch, while inner and outer elements can overrun or freewheel in the opposite direction. Nearly all modern passenger car automatic transmissions use one or more one-way clutches to smooth transitions from one torque path to another as the transmission shifts through its operating ranges. Also known as overrunning clutches or freewheels, the most common one-way clutch types are:    1) Roller one-way clutch    2) Sprag one-way clutch    3) Ratchet one-way clutch
The roller and sprag types have been in production since the early 1950's. Production of ratchet clutches started in the mid 1990's. All of these one-way clutch assemblies work satisfactorily depending upon the particular application in which they are used. The basic construction of all these clutches includes an outer race, an inner race, a set of locking elements (rollers, sprags or pawls) that can interlock the races, and springs that activate the locking elements.
Initially, races for roller and sprag one-way clutches were manufactured from wrought steels following traditional machining methods (turning, broaching etc.). This process remains in production today. Typical steels used are SAE 1060, SAE 5060, SAE 5120, SAE 5130 and SAE 5160. In order to develop the necessary contact fatigue strength at the race surface and tensile fatigue strength in the bulk of the race, the parts are typically heat treated to HRC 55-60 on the surface and HRC 25-32 in the core.
A major improvement to the manufacturing of races was introduced in U.S. Pat. No. 3,772,935. The disclosed process involves compacting and sintering powder metal into a preform blank and then hot forging the preform to shape the component to yield a full, uniform density throughout the race. Finally, the races are heat-treated. The key advantage of the powder forging process is that it reduces machining. Improvements to the heat treatment step of this process are shown in U.S. Pat. Nos. 3,992,763 and 4,002,471.
An alternative powder forging process is discussed in U.S. Pat. No. 5,966,581. The compacted and sintered preform is worked with a roller-burnishing tool prior to the forging step. The roller burnishing process coupled with the forging is said to increase the fatigue life of the final hot forged, fully densified clutch race.
Another alternative discussed in U.S. Pat. No. 5,966,581 is to cold work the compacted and sintered preform by blasting the surface with glass beads or sand which works the surfaces, followed by hot forging of the preform. This approach results in a comparably rough surface finish than that attained by roller burnishing.
Accordingly, while it is recognized in the art to fabricate races of one-way clutch mechanisms from powder metal that has been compacted and sintered into a preform blank which is roller burnished and then hot forged to yield a fully densified component of desired shape, the art teaches against the possibility of fabricating a net-shape compacted and sintered powder metal clutch race component whose final surface is cold worked that would have suitable mechanical properties and surface finish for its intended use.
It is an object of the present invention to advance the art by improving the processing of one-way clutch races made of powder metal.